High-Speed, Low-Distortion,

Differential Line Receivers

12

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be a resistor, or it can be a complex pole-zero pair for

filter and shaping applications (Figure 9). Use surface-

mount gain-setting components to ensure stability.

Using REF and SENSE

The MAX4144/MAX4145/MAX4146 have a REF pin (nor-

mally connected to ground) and a SENSE pin (normally

connected to OUT). In some long-line applications, it

may be desirable to connect SENSE and OUT together

at the load, instead of the typical connection at the part

(Figure 3). This compensates for the long line’s resis-

tance, which otherwise leads to an IR voltage error.

When using this technique, keep the sense lines’

impedance low to minimize gain errors. Also, keep

capacitance low to maximize frequency response. The

gain of the MAX4144/MAX4145/MAX4146 output stage

is approximated by the following equation:

trace impedances, respectively. R is 700Ω for the

MAX4144 and MAX4145, and 100Ω for the MAX4146.

Additionally, mismatches in the SENSE and REF traces

lead to common-mode gain errors. Common-mode

gain is approximated by the following equation:

Driving Capacitive Loads

The MAX4144/MAX4145/MAX4146 provide maximum

AC performance when not driving an output load

capacitance. This is the case when driving a correctly

terminated transmission line (i.e., a back-terminated

cable). In most amplifier circuits, driving large load

capacitance increases the chance of oscillations. The

amplifier’s output impedance and the load capacitor

combine to add a pole and excess phase to the loop

response. If the pole’s frequency is low enough and

phase margin is degraded sufficiently, oscillations may

occur. A second concern when driving capacitive

loads results from the amplifier’s output impedance,

A=

R

-

R

R + 700

VCM

REF

SENSE

ΔΔ

Ω

700

R

R

700

R

REF

REF

ΩΔ

ΩΔ

+

+

++

⎤

⎦

⎥

A=

1

2

700

R

R

1

700

R

R

700

R

V

SENSE

REF

REF

ΩΔ

ΩΔ

ΩΔ

+

+

+

++

⎛

⎝⎜

⎞

⎠⎟

⎡

⎣

⎢

G = A = 1 +

1.4k

R

(MAX4145)

G = A = 10 +

14k

R

(MAX4146)

V

G

V

G

Ω

Ω

100k

1M

10M

100M

1G

-3

-4

-5

-2

FREQUENCY (Hz)

-1

0

1

2

3

4

5

Figure 4. MAX4144 Small-Signal Response with Capacitive

Load

SENSE

OUT

REF

MAX4144

MAX4145

MAX4146

Figure 3. Connection of SENSE and REF to a Remote Load

IN-

IN+

MAX4145

MAX4146